The novel feature which attracted the most attention and scepticism regarding this design was--so I was later informed by a member of the Board--in the claim made that the vessel could readily navigate over the water-bed itself and that while navigating on the water-bed a door could be opened in the bottom of a compartment and the water kept from entering the vessel by means of compressed air, and that the crew could, by donning diving suits, readily leave and enter the vessel while submerged. Another novel feature was in the method of controlling the depth of submergence when navigating between the surface and the water-bed. The vessel was designed always to submerge and navigate on a level keel rather than to be inclined down or up by the bow to dive or rise. This maintenance of a level keel while submerged was provided for by the installation of four depth-regulating vanes, which I later termed "hydroplanes" to distinguish them from the forward and aft levelling vanes or horizontal rudders. These hydroplanes were located at equal distances forward and aft of the centre of gravity and buoyancy of the vessel when in the submerged condition, so as not to disturb the trim of the vessel when the planes were inclined down or up to cause the vessel to submerge or rise when under way. I also used, in conjunction with the hydroplanes, horizontal rudders, which I called "levelling vanes," as their purpose was just the opposite from that of the horizontal rudder used in the diving type of vessel. They were operated by a pendulum-controlling device to be inclined so as always to maintain the vessel on a level keel rather than cause her to depart therefrom. When I came to try this combination out in practice I found hand control of the horizontal rudders was sufficient. If vessels with this system of control have a sufficient amount of stability, they will run for hours and _automatically maintain both a constant depth and a level keel_, without the depth-control man touching either the hydroplane or horizontal rudder control gear. This automatic maintenance of depth without manipulating the hydroplanes or rudders was a performance not anticipated or claimed in my original patent on the above-mentioned combination, and what caused these vessels to function in this manner remained a mystery, which was left unsolved until I built a model tank in 1905, in Berlin, Germany, and conducted a series of experiments on models of submarines. I then learned that the down pull of a hydroplane with a given degree of inclination varied according to its depth of submergence, and the deeper the submergence the less down pull. This works out to give automatic maintenance of depth so long as the vessel is kept at a constant trim on a substantially level keel, and I have known of vessels running for a period of over two hours without variation of depth of one foot and without once changing the inclination of either the hydroplanes or the horizontal rudder.